Thermoelectric material and devices



Nov. 21, 1961 M. D. HOUSTON THERMOELECTRIC MATERIAL AND DEVICES Filed Aug. 14, 1959 2 Sheets-Sheet 1 Fig. l.

Samarium or Rare Earth Subsulfide Fig.2.

Maurice D. Hous'ron.

BY WM ATTQR EY Nov. 21, 1961 M. D. HOUSTON THERMOELECTRIC MATERIAL AND DEVICES Filed Aug. 14, 1959 2 Sheets-Sheet 2 ,Fig.3.

0 7 o l l 2 m a w 0 0| 1 l I M A 0 7 o I l 2 m 0 0| W 0 7 O 1 2 H 8 0 0| w o 7 2 w s 0 0! I l l 8 m m 7 4 0 0| l I l 6 w w 5 2 0 l 4 O ||||l| I l I ||.|||||..|.|||||||l|||||||l||||].|I.||||||7 w 2 0 1 2 o w l O r .I. 4 2 06 4 2 O 0 O O 4 2 O \I O O O \l O \I 2 l I m V TC 4 c5 PC n KO O 0 am P n w c ZX (X 1 P K United States atent fifice 3,009,977 Patented Nov. 21, 1961 3,009,977 THERMOELECTRIC MATERIAL AND DEVICES Maurice D. Houston, Monroeville, Pa., assiguor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 14, 1959, Ser. No. 833,773 9 Claims. (Cl. 1365) The present invention relates generally to thermoelectric elements and thermoelectric devices embodying the same, and more particularly to thermoelectric elements comprised of rare earth subsulfide compounds.

It has been regarded as highly desirable to produce thermoelectric devices wherein either an electric current is passed therethroughto provide for cooling applications, or alternatively a source of heat is applied to one junction of a thermoelectric device to bring this junction to a given elevated temperature while the other junction is kept at a low temperature, whereby an electrical voltage is generated in the device. For refrigeration or heat extraction applications in particular, one junction of the thermoelectric device is disposed within an insulated chamber and electrical current is passed through the junction in such a direction that the junction within the chamber becomes cooler while the other junction of the thermoelectric device is disposed externally of the chamber and dissipates heat to a suitable heat sink such as the atmosphere, cooling water or the like.

- When heat is applied to one junction of a thermoelectric device while the other junction is cooled, an electrical potential is produced proportional to the thermoelectric power of the thermoelectric elements employed, and to the temperature difference between the junctions. Accordingly, it is desirable that the thermoelectric elements be made of such material, all other factors being equal, that the highest potential is developed for the temperature difference between the hot and cold junctions. The electrical resistivity of the thermoelectric element member of the device and the thermal conductivity of the element both should be as low as possible inorder to reduce electrical losses and thermal losses.

Thermoelectric devices may be tested and a number indicating its relative effectiveness, called the figure of merit, may be computed from the test data. The higher the figure of merit, the more eflicient is the thermoelectric design. The figure of merit, denoted as Z, is defined by:

2 :gK wherein S=Seebeck coeflicient (volts/ C.) =electrical resistivity (ohm-cm.)

' K=thermal conductivity (watts/cm. C.)

wherein T isthe absolute temperature and the other symbols have the meaning set forth above.

An object of'the present invention is to provide a thermoelectric power generating device in which the one element is comprised of a material having the formula MS Se wherein M is an element of the lanthanum rare earth series.

Another object of the present invention is to provide a thermoelectric power generating device in which one element is comprised of a material having the formula MS A further object of the present invention is to provide a thermoelectric power generating device in which one element is comprised of a material having the formula ro to 0.5-

A still further object of the present invention is to provide a process for producing a thermoelectric material comprised of a lanthanum series rare earth subsulfide comprising, admixing the rare earth metal and sulfur in a ratio such that the ratio of metal to sulfur is at least one, reacting the metal and the sulfur at an elevated tem perature, cooling the reacted mass to ambient temperature, compacting the reacted material into the desired configuration, and firing the compacted material at an elevated temperature. v

Other objects of the present invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawings in which:

FIGURE 1 is a schematic view partly in cross section of a thermoelectric power generating device.

FIG. 2 is a schematic view partly in cross section of a second thermoelectric power generating device; and,

FIG. 3 comprises graphs plotting the temperature against various properties of the thermoelectric material of this invention.

This invention is directed to the preparation and use of certain rare earth metal subsulfide compounds as thermoelectric element members in thermoelectric power generating and heat extraction or refrigeration-devices.

In accordance with the present invention and attainment of the foregoing objects, there is provided a thermoelectric power generating device comprising at least one pair of joined members, one member of each pair comprised of a material having the formula MS Se wherein M represents at least one element selected from the lanthanum rare earth series or-group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbiurn, and lutecium and y varies from 1.0 to 0.5, and x varies from 0 to" 0.2.

In accordance with the present invention there is provided a process for producing the thermoelectric material having the equation MS Se comprising admixing the reactants M, S and Se in a finely divided particle form, reacting the admixed materials at an elevated temperature, cooling the reacted mass to ambient temperature,

compacting the cooled reactant material to the desired configuration and thereafter firing the compact at an elevated temperature.

The most convenient and satisfactory process for preparing the material of thisinvention comprises admixing in finely divided particle form at least one element (M) selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutecium, with the'requi'red amount of sulfur.

The element (M) should have'a purity of at least 99%, and preferably 99.6% and higher and the sulfur a purity of at least 99.5% for the most satisfactory results. The nature of the impurities will determine the amounts permissible. The materials should be admixed in such proportions so thatv the element to sulfurratio isat least one. Preferably the materials shouldrbe in a finely divided particle form such that all of the particles will 3 pass through a 120 mesh (US. standard) sieve. If desired, a doping material, selenium, may be admixed with the element and the sulfur. The selenium should have a purity of at least 99.9%, and have a particle size such that the entire mass will pass through a 120 mesh (U.S. standard) sieve. The materials should be admixed in sufficient quantities to provide a reacted compound having the formula MS Se wherein y varies from 1.0 to 0.5 and x varies from to 0.2. The reactants are admixed to a state of good homogeneity.

The admixed reactants are then charged into a bulb or tube of an inert material, for example a Vycor or quartz tube and sealed therein under a vacuum of at least 4X mm. Hg.

The reactants are then heatedtoa temperature vwithin the range of 626 C. .to 800 C. at a rate of up to 250 C. per hour and maintainedat thisrelevated temperature for a period of time ranging from ten minutes to two hours.

If the metal-sulfur admixture is heated to'a temperature of less than 626 little or no reaction will take place. If the materials, are heated to a temperature ticularly satisfactory when operating in a temperature range of from 425 C. to 1000 C. in a protective materially in excess of 800 C. the resultant-reaction 7 product will not have the best thermoelectric properties.

Very satisfactory results have been achieved'when the metal-sulfur admixture, and selenium if present, have been heated to a temperature of from 630 C. to 750 C.

Care must be taken in controlling the rateat which the reactants are heated from the ambient .to the. desired reaction temperature. Satisfactory results vhave been achieved when the temperaturehas been elevated .at a rate of from 150 C. to 250 0.. per hour. A rate of less than 150 C. per hour is commercially impractical. If the temperature is elevated at a rate substantially greater than 250 C. per hour, the reaction, which is, exothermic, will generate so much hcatthat the inert vessel may melt or be blown up. A rate of 200 C. per-hour has been found quite satisfactory.

The reaction has-been found to begin almost immediatelyupon reaching a temperature of 626, however, the reaction should) be allowed to continue for at least ten minutes to ensure completion. If allowed to. react in excess of two hours the re'actioniproducts ihavc'been found to have less satisfactory thermoelectric properties;

After; reacting,' the subsulfide product is allowed to cool to the ambient temperature. The rate at which the reactionproduct is cooledis-not critical and satisfatcory results have been achieved when the reaction vessel has been removed; from the furnace and allowed to establish thermodynamic equilibrium withthe ambientat an uncontrolled but natural rate.

The'reaction'product is a finely'divided powder, usually a powder having a substantially black color. The-powder is groundinto finely divided particle's such that all particles will pass, through a 120 mesh (U.S.' standard) sieve. a

. The. finely divided particles offthe subsulfide reaction products are then charged into a suitable mold and compacted at room temperature to the desired configuration. A pressure of from-5 to 100 tons per square inch, preferably tons per square inch, hasbeen found satisfactory.

Satisfactory results have been achieved whenthev compaction is carried out using; a; tungsten carbide mold. V

The compacts are, then charged into a furnace and f fired under avacuum'of about 4 1,0" mm. Hg, at. a

temperature within the range-of 900C.'to 1600jC. for a period of time varying from fifteen minute s t'o'one" hour. Satisfactory results have been achievedin-firing" the I compacts at'a temperature or l200' C. "for thirty;

minutes; The compacts haven-type thermoelectric characteristics i The compactsthusprepared' are nowready to have electrical contacts aflixed thereto and to be employed as thermoelectric elements in a; thermoelectric: power gencrater: j The reaction products of- -invention are paratmosphere, for example in a vacuum or an inert atmosphere such as argon, heliumor nitrogen atmosphere. V In air the thermoelectric material will oxidize at high ten'iperature and lose its thermoelectric efiiciency.

Referring to FIGURE 1 of the drawing,

an electrical current'from a heat'source. A thermally insulating wall 10 so formed as 'to providesuitable furnace chamber or other thermal barrier is perforated to permit passage therethrough of a positive thermoelectric member 12, and a negative thermoelectric element 14 comprised of thematerial of thisinvention and having the formula MS Se in which M represents at least one element selected from the group consisting of lanthanum, cerium, praseodymiurn, neodymium, ,samarium, europium, gadolinium, terbium, dysprosium,i ;holmi-' um, erbiurmthulium, ytterbium, and lutecium, and y varies from 1.0 to 0.5, a'nd x-varies from 0 to 0.2. An

electrically conducting strip 16 comprised of a suitable metal, for example, copper, silver palladium or platinum or, the like is joined to anend'face of a member 12 and end face of the member 14 within the chamber so as to provide good electrical'and thermal contact there with. The strip 16 must be comprised of ametal that trical contact is obtained. The metal strip 16' maybe brazed or. soldered to the metal layers 18 and 20. The

metal strip 16 may be provided with suitable finsor other extended surfacemeans (not shown) for conducting heat efficiently thereto from the. furnace chamber or other heat source to whichit is exposed.

At the end of the-member 12.-located on the other side 7 of wall 10 a metal plate or strip 22 is attached by braz ing or soldering in the same manner 'as was employed in attaching strip. 16 to the other end face. Similarly, 'a metal strip or plate 24 may be connected to the. other end of member 14. The plates 22 and 24 may be-provided with heat dissipating fins or other. cooling means wherebyheat conducted thereto may be dissipated.,-'[he surfaces of the plates 22 and 24 mayalso. be cooled by passing a current of fluid such. as airor water across them. An .electricalconductor 26' in circuit with a load I 28 is electrically connected to the plates 22and'24. A switch 30 is interposed in the conductor 26 to enable thev electrical circuit to be opened and :closed as-desired. When switch 30 is moved tov the closed position, an elec .trical'current flows between members 12 and 14 andenergizesthe load 28.

1 It will be appreciated that a plurality of pairs of the positive and negative members may be joined in series in; 1 order to produce a plurality of cooperating thermalele I ments to provide a desired potential. Each of the thermal elements will be disposed with one 'junctionin a furnace or exposed to another. source of heat .while' the f'other' junction is cooled by applying'wateror blowing air thereon or the like. Due tome-relative difference'in temperature of the junctions, an electricalpotential will be gen'eratedin each ofthethermal elements. Byjoinin'gI in series .a suitable number offthe thermalel emen'tsg direct' current at any .suitable'voltage will be generated. When employing the rnaterial of this invention'in a thermoelectric power generating device two'problems are' 1 l presented; (l) pairing'up the n-type subsulfide elementis with p-type elements having a figureoffmerit 'vvithin the "temperature range of the material of; invention; and" (2 ipreventinggthe' oxidation of the jsubsulfid ma w app oac ng.

tefial 70f ibis. n n ion at temPeratur IQOQY- V there is illustrated a thermoelectric device suitable for" producing With reference to FIG. 2, there is illustrated a thermoelectric device designed to overcome both of the above problems. N-type thermoelectric elements, comprised of the material of this invention and having the formula MS,,. Se and having platinum or palladium electrodes 42 disposed at each end thereof, are joined or connected together in the manner illustrated by copper straps 44. The elements 40 are disposed within a chamber 46. The chamber 46 has perforations through the Walls thereof to permit the passage of the contacts 42 therethrough. A gas inlet passage 48 and a gas exit passage 50 are dis posed at opposite ends of the chamber 46. A load 51 is connected through a conductor 52 to one of the thermoelectric elements 40 at point 56 and the circuit is completed by passing the conductor 52 through the chamber wall 46 at a point 56 and making contact to a copper strap at point 58. A switch 60 is disposed in the circuit to control the energization of the circuit. During the operation of the thermoelectric power generator of FIG. 2, an inert gas, for example nitrogen, helium, argon or mixtures thereof and the like is passed into chamber 46 through the inlet 48 and withdrawn through the exit 50. Heat is applied to the lower side of the chamber 46, thereby heating the element ends disposed through that side of the chamber to an elevated temperature. The opposite or top ends of the thermoelectric elements 40 are cooled in any suitable manner known to those skilled in the art. As described above, the temperature difference initiates a potential difference which causes the flow of an electric current within the n-type elements 40. The direction of the current flow is indicated by the arrows 62. When the switch 60 is moved either to the open or to the closed position, an electrical current flows through the circuit and energizes the load 51, or is interrupted.

The following examples are exemplary of the teachings of this invention.

EXAMPLE I 150.35 grams of Samarium metal having a purity of 99.6%, the balance being other rare earths, and 24.05 grams of sulfur having a purity of 99.9% were admixed. The samarium and sulfur were in the form of particles all of which would pass through a 120 mesh (U.S. standard) sieve. The samarium and sulfur were thoroughly admixed.

The homogeneous admixture was then charged into a Vycor tube and sealed therein under an absolute pressure of 4' 10 millimeters of mercury. The Vycor tube was then charged into a furnace and the sarnarium and sulfur heated to a temperature of 800 C. at a rate of approximately (but not exceeding) 250 C. per hour. The sarnariurn and sulfur were maintained at a temperature of 800 C. for approximately thirty minutes during which time a chemical reaction took place. The Vycor tube with the reacted Samarium sulfide (SmS was removed from the furnace and allowed to cool to room temperature. l

. After reaching room temperature, the reacted samariurn sulfide was removed from the Vycor tube and ground, with a mortar and pestle, to a size such that the entire mass of black powder would pass through a 120 mesh (U.S. standard) sieve.

The SmS Was then charged into a series of tungsten carbide molds and compacted with a pressure of 50 tons per square inch into a series of cylindrical pellets having a diameter inch and height of /2 inch.

. The pellets thus prepared were fired in a protective atmosphere at va-rious'temperatures ranging from 800 C. to 1300' C. for a period of thirty minutes.

' The electrical properties of the pellets fired at 1200 C. for thirty minutes were determined and are set forth graphically in FIG. 3. The thermal conductivity (K) values were calculated. From the graphical presentation of the figure of merit it can be seen that the material thus prepared had an etficiency ranging from 7 /z% to 10% 6 within the temperature range of 425 C. to 1000 C. A peak efficiency of 10% was reached at a-temperature of approximately 725 C.

The electrical properties of the various samples of 5 SmS prepared in the manner set forth above are presented in table form below.

Table I ELECTRICAL AND THERMOELECTRICAL PROPERTIES OF SmSnJa If the thermal conductivities of the samples are considered to be the same, the quantity 8 P can be taken as a rela-' tive thermoelectric value of the material. The firing temperature is the temperature at which the compacts were fired.

EXAMPLE II The procedure of Example I was followed using 150.35 grams of samarium and 16.03 grams of sulfur. The electrical and thermoelectric properties of the compound SmS thus prepared are set forth in tabular form below.

Table II ELECTRICAL AND THERMOELECTRICAL PROPERTIES OF sms j Electrical Seebeck Firing Temperature C.) Resistivity Coeigcient S IP (ohm-cm.) v./ O.)

800 1. l0 10- -98 8 72 l0' 5. 95X10- 70 8 24X10 2. 41 1O 72 2 15X10" 2. 89 10- 71 1 74 l0- 3.1?)(10- 85 2 28 10 6. 20 10- 89 1 28x10- EXAMPLE III Table III ELECTRICAL AND TrrnaMg igr no 'rnroaL PROPERTIES Electrical Seebeck Firing Temperature 0.) Resistivity Coetfieicnt S /P p (u (ohm-cm.)

2.6X10- 238 2 18 10- 9 2X10" -245 6 53 1O- 9.8X10- 347 1 23x10- 9.9 10- 314 9 96 10- 1. 1X10- 337 1 03X10 3. 0X10 174 1 0l 10- 6. 0X10" 204 6. 95X10 1. 6 10- 226 3. 19x10 1. 0 10- --275 7. 56X10- EXAMPLE IV 138.92 grams of lanthanum and 16.03 grams of sulfur were admixed to form a homogeneous mixture. The ad:

mixture was made up of particles of lanthanum and sulfur of such a size that they would all pass through a 120 mesh (U.S; standard) sieve.- The admixture was charged into a Vycor tube and sealed under a high vacuum. The tube was charged into a furnace and .heated'to a temperature of 750 at a rate of 150 C. per hour.: The lanthanum and sulfur was maintained at the reaction temperature of 750 for one hour and then removed from the furnace and allowed to cool to room temperature.

- The react-ion product was then compacted into pellets one inchlong' and having a diameter of /2 inch under a pressure of 100 tons per square. inch. The comp-act thus formed was fired at a temperature of 1200 C. for one hour.

The material LaS thus prepared is suitable for use for thermoelectric purposes.

EXAMPLE VI 167.27 grams of erbium and 32.06 grams of sulfur in finely divided particle form were admixed until a homogeneous mixture was obtained. The admixture was sealed in a Vycor tube and heated to a reaction temperature of 630 at a rate of 200 C. per hour. After remaining at the reaction temperature for a period of approximately ten minutes the Vycor tube was removed from the furnace and the reaction product-ofthe erbium and. sulfur allowed to cool to room temperature. The reaction prodnot of the erbium and sulfur had the formula ErS and was in the form of a finely divided powder.

The ErS powder was compacted into-pellets having a diameter ,of approximately 1%: inch and a length of dysprosium, holmium, erbium, thulium, ytterbium, and lutecium, y varies from 1.0 to 0.5 and x varies from 0 to 0.2, and another suitable member electrically connected to one portion of said one member.

2. In a thermoelectric power gene'ratingdevice at least members, one beinga member corn- 7 one pair of joined prised of a material having the formula Ms wherein M represents at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neo dymium, Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and luteci um and y varies from 1. to 0.5, and anothersuitable mem: ber electrically connected to one portion of said one member. I

3. In a thermoelectric power generating device at least one pair of joined members, one member comprised of a material having the formula SmS Se wherein y varies from 1 to 0.5 and x variesfrom 0 to 0.2, andthe other memberof the pair electrically connected to-orie portion of said one member.

4. In a thermoelectric power generating device at least one pair of 'joined members, one member comprised of a material having the formula SmS 1 05 and theother member of the pair electrically connected to one portion of said one member.

5. In a thermoelectric power generating device at least one pair of joined members, one member comprised of a material having the formula SmS and the other mernber of the pair electrically connected to one portion of said one'member. V t f p 6. A process for preparing a thermoelectric" material having a formula MS Se ,-wherein M is 'at least one metal selected from the group consisting of lanthanum,

cerium, praseodymium, neodymium, Samarium, europi.-'

urn, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutecium and y varies from lto 0.5 and x'varies from'O to 0.2, comprising (1 admixing 'predetermined quantities of sulfur with at least one of the aforesaid metals and from 0 to 20 mol percent selenium in finely divided particle form, (2) heatingjsaid mateapproximately 1 inch with a pressure of 5 tons per square inch. The pellets thus formed were. fired at a temperature of approximately 900. C. for twenty minutes. The compound. in the pellets thus'formed had the formula BS and is suitable for thermoelectric applications,

Satisfactory results will be achieved in practicing the process set forth in Example I and substituting cerium, praseodymiurn, neodymium, europium, gadolinium, terbium, dyspr osium, holmium, thulium, ytterbium, and.

rial in a vacuum at a rate up to 250 C. per hour to an elevated reaction temperature in excess of 625 C (3) maintaining said elevated reaction temperature" for-"Tatleast ten minutes, (4) cooling ther'eacte'drna'terial to room temperature. a

- 7. A process for preparing a thermoelectric material having the-formula MS Se wherein M isatla'st one procedureset forth in Example I to produce athermox electric, material having the formula SmCeS. The ther- 'moelectricj material thus prepared'indicated it has satisfactory thermoelectric properties. i

It will be understood that mixtures of two ormore of the rare earths M may be combined with sulfur .or su1-' fur and selenium as set forth herein.

While the invention has been described withrreference to particular embodiments and "examples, it will be understood thatkmodifications, substitutions and the like may be made therein without departing from its scope.

I claim as my invention: v 1. In ii-thermoelectric power 'gener'ating'device at least one pairof joined members, one-being a member 'com- 3 prisedofamaterial having the formula MS g 'Se where-;

in M represents-catv least one element selected from the group consisting. of lanthanum, cerium, praseodym'ium,

neodymium, samarium,v europium," gadolinium, terbium,

metal selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, Samarium, euro pi um, gadolinium, 'terbium, dysprosium, holmium, er bium, thulium, ytterbium and lutecium and y varies from 1 to 0.5 and x varies from (H002, comprisingil) admixing predetermined quantities of. sulfur :with at least one of the aforesaid'metals and from 071620 mol percent selenium in finely divided particleform, (2)

heating said materialin' a vacuum at a rate of fror'nj15 0 C. to250 C. per hour to a temperature within the range" of. 625 c. to 300 0., (3 maintaining 's'aid' elevated 7 temperaturefor a period of time ranging from ten minutes to two hours, and (4) cooling theireactedmaterial to room temperature.

8AA process forpreparing a pellet of a thermoelectric. 7 material having a formula MS;', Se ,--wherein M is at 3" least one metal' sel'ectedkfrom the group consisting of;,

lanthanum, cerium, pra'seodymiu'm, neodymium; Samariurn, europiurn, gadolinium; terbium, dysprosium, holmium, erbiumjthuliurn, ytterbium,andlutecium and y.

equals 1' to 0.5 and t" varies from 0 'to 0.2,'comprising ,(1) admixing predetermined quantities of sulfur with at least one of the aforesaid metalsland from i0 to20 mol percent 7 selenium in finely divided particle form,;(2) heating said material in a vacuurri at 'a rateofup to 250" C. perh'our v tOljan elevated reaction temperature in -'exc'e'ss of 625C (3.), maintaining said elevated temperature forgatgleast'ten minutes, -(4) cooling the reacted matcrial tolroo'm ternperature, (5 compacting-saidreacted (material into; i

compact with a pressure within the range of from 5 to 100 tons per square inch, and (6) firing in a vacuum at a temperature of from 900 C. to 1600 C. for from onequarter hour to one hour.

9. A process for preparing a pellet of a thermoelectric material having a formula MS Se wherein M is at least one metal selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutecium, y equals 1 to 0.5 and 2: equals 0 to 0.2, comprising (1) admixing predetermined quantities of sulfur with at least one of the aforesaid metals and from 0 to 20 mol percent selenium in finely divided particle form, (2) heating said material in a vacuum at a rate within the range of from 150 C. to 250 C. per hour to an elevated reaction temperature Within the range of 625 C. to 800 C., (3) maintaining said elevated temperature for a period of time within the range of from ten minutes to two hours, (4) cooling the reacted material to room temperature, (5) compacting said reacted material into a compact with a pressure of from 5 to 100 tons per square inch, and (6) firing in a protective atmosphere at a temperature of from 900 C. to 1600 C. for a period of time ranging from one-quarter hour to one hour.

References Cited in the file of this patent UNITED STATES PATENTS 685,471 Hermite et a1 Oct. 29, 1901 1,613,877 Dyckerhotf Jan. 11, 1927 2,811,570 Karrer Oct. 29, 1957 

1. IN A THERMOELECTRIC POWER GENERATING DEVICE AT LEAST ONE PAIR OF JOINED MEMBERS, ONE BEING A MEMBER COMPRISED OF A MATERIAL HAVING THE FORMULA MSY-XSEX, WHEREIN M REPRESENTS AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF LANTHANUM, CERIUM, PRASEODYMIUM, NEODYMIUM, SAMARIUM, EUROPIUM, GADOLINIUM, TERBIUM, 